1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, the present invention relates to a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a driving method thereof, or a manufacturing method thereof, for example. Furthermore, the present invention relates to a semiconductor device, a display device, or a light-emitting device each including a transistor, or a driving method thereof, for example. The present invention relates to an electronic appliance or the like including the semiconductor device, the display device, or the light-emitting device.
Note that in this specification, a semiconductor device refers to any device that can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, an electronic appliance, and the like are all included in the category of the semiconductor device.
2. Description of the Related Art
Attention has been focused on a technique for forming a transistor using a semiconductor film which is formed on a substrate having an insulating surface. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. A silicon film is known as a semiconductor film applicable to a transistor.
As the silicon film used as a semiconductor film of a transistor, either an amorphous silicon film or a polycrystalline silicon film is used depending on the purpose. For example, in the case of a transistor included in a large-sized display device, it is preferred to use an amorphous silicon film, which can be formed using the established technique for forming a film on a large-sized substrate. On the other hand, in the case of a transistor included in a high-performance display device where driver circuits are formed over the same substrate, it is preferred to use a polycrystalline silicon film, which can form a transistor having a high field-effect mobility.
In recent years, a transistor that includes an oxide semiconductor film containing indium, gallium, and zinc has attracted attention.
An oxide semiconductor film can be formed by a sputtering method or the like, and thus can be used for a transistor included in a large display device. Moreover, a transistor including an oxide semiconductor film has a high field-effect mobility; therefore, a high-performance display device where driver circuits are formed over the same substrate can be obtained. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including an amorphous silicon film can be retrofitted and utilized.
As a method for providing a transistor including an oxide semiconductor film with stable electrical characteristics, a technique where an insulating film containing excess oxygen is used as an insulating film in contact with an oxide semiconductor film is disclosed (see Patent Document 1). The technique disclosed in Patent Document 1 enables oxygen vacancy in an oxide semiconductor film to be reduced. As a result, variation in electric characteristics of the transistor including the oxide semiconductor film can be reduced and reliability can be improved.